Systems for a radio frequency coil for MR imaging

ABSTRACT

Various methods and systems are provided for a flexible, lightweight, and lowcost radio frequency (RF) coil of a magnetic resonance imaging (MRI) system. In one example, a RF coil assembly for an MRI system includes a distributed capacitance loop portion comprising at least three distributed capacitance conductor wires encapsulated and separated by a dielectric material, a coupling electronics portion including a preamplifier, and a coil-interfacing cable extending between the coupling electronics portion and an interfacing connector of the RF coil assembly.

CROSS REFERENCE

This application claims benefit and priority to U.S. ProvisionalApplication No. 62/590,241 filed on Nov. 22, 2017, the entirety of whichis incorporated herein by reference.

FIELD

Embodiments of the subject matter disclosed herein relate to magneticresonance imaging (MRI), and more particularly, to MRI radio frequency(RF) coils.

BACKGROUND

Magnetic resonance imaging (MRI) is a medical imaging modality that cancreate images of the inside of a human body without using x-rays orother ionizing radiation. MRI systems include a superconducting magnetto create a strong, uniform, static magnetic field. When a human body,or part of a human body, is placed in the magnetic field, the nuclearspins associated with the hydrogen nuclei in tissue water becomepolarized, wherein the magnetic moments associated with these spinsbecome preferentially aligned along the direction of the magnetic field,resulting in a small net tissue magnetization along that axis. MRIsystems also include gradient coils that produce smaller amplitude,spatially-varying magnetic fields with orthogonal axes to spatiallyencode the magnetic resonance (MR) signal by creating a signatureresonance frequency at each location in the body. Radio frequency (RF)coils are then used to create pulses of RF energy at or near theresonance frequency of the hydrogen nuclei, which add energy to thenuclear spin system. As the nuclear spins relax back to their restenergy state, they release the absorbed energy in the form of an MRsignal. This signal is detected by the MRI system and is transformedinto an image using reconstruction algorithms.

As mentioned, RF coils are used in MRI systems to transmit RF excitationsignals (“transmit coil”), and to receive the RF signals emitted by animaging subject (“receive coil”). Coil-interfacing cables may be used totransmit signals between the RF coils and other aspects of theprocessing system, for example to control the RF coils and/or to receiveinformation from the RF coils. However, conventional RF coils tend to bebulky, rigid and are configured to be maintained at a fixed positionrelative to other RF coils in an array. This bulkiness and lack offlexibility often prevents the RF coil loops from coupling mostefficiently with the desired anatomy and make them very uncomfortable tothe imaging subject. Further, coil-to-coil interactions dictate that thecoils be sized and/or positioned non-ideally from a coverage or imagingacceleration perspective.

BRIEF DESCRIPTION

In one embodiment, a radio frequency (RF) coil assembly for a magneticresonance (MR) imaging system includes a loop portion comprising atleast three wires encapsulated and separated by a dielectric material,the at least three parallel wires form distributed capacitance along thelength. The RF coil assembly also comprises a coupling electronicsportion including a pre-amplifier and a coil-interfacing cable extendingbetween the coupling electronics portion and an interfacing connector ofthe RF coil assembly. In this way, a flexible RF coil assembly may beprovided that allows for RF coils in an array to be positioned morearbitrarily, allowing placement and/or size of the coils to be based ondesired anatomy coverage, without having to account for fixed coiloverlaps or electronics positioning. The coils may conform to thepatient anatomy, rigid or semi-rigid housing contours with relativeease. Additionally, the cost and weight of the coils may besignificantly lowered due to minimized materials and production process,and environmentally-friendlier processes may be used in the manufactureand miniaturization of the RF coils of the present disclosure versusconventional coils. Further, by including at least three distributedcapacitance wires, the capacitance of the RF coil assembly may beincreased relative to RF coils comprised of fewer parallel wires (e.g.,two parallel wires), allowing for usage of the RF coil assembly in an MRenvironment where lower resonance frequency is desired.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 is a block diagram of an MRI system according to an exemplaryembodiment.

FIG. 2 schematically shows an example RF coil coupled to a controllerunit.

FIG. 3 shows a first example RF coil and associated couplingelectronics.

FIG. 4 shows a second example RF coil and associated couplingelectronics.

FIG. 5 shows a cross-sectional view of the loop portion of the RF coilof FIGS. 3 and 4.

FIG. 6 shows a loop portion of a third example RF coil.

FIG. 7 shows a cross-sectional view of the loop portion of FIG. 6.

FIG. 8 shows a loop portion of a fourth example RF coil.

FIG. 9 shows a cross-sectional view of the loop portion of FIG. 9.

FIG. 10 is a graph illustrating ascending and descending current.

FIG. 11 shows a cross-sectional view of a loop portion of a fifthexample RF coil.

FIG. 12 shows a cross-sectional view of a loop portion of a sixthexample RF coil.

FIG. 13 shows a cross-sectional view of a loop portion of a seventhexample RF coil.

FIG. 14 shows a plurality of example RF coil array configurations.

FIG. 15 schematically shows an example RF coil array interfacing cableincluding a plurality of continuous and/or contiguous common mode trapspositioned between a processing system and a RF coil array of a MRIsystem.

FIGS. 16 and 17 schematically show example RF coil array interfacingcable including a plurality of continuous and/or contiguous common modetraps.

DETAILED DESCRIPTION

The following description relates to various embodiments of a radiofrequency (RF) coil in MRI systems. In particular, systems and methodsare provided for a low-cost, flexible, and lightweight RF coil that iseffectively transparent in multiple respects. The RF coil is effectivelytransparent to patients, given the low weight of the coil and flexiblepackaging that is enabled by the RF coil. The RF coil is alsoeffectively transparent to other RF coils in an array of RF coils, dueto minimization of magnetic and electric coupling effects. Further, theRF coil is effectively transparent to other structures throughcapacitance minimization and is transparent to positrons through massreduction, enabling use of the RF coil in hybrid positron emissiontomography (PET)/MR imaging systems. The RF coil of the presentdisclosure may be used in MRI systems of various magnetic fieldstrengths.

The RF coil of the present disclosure uses a significantly smalleramount of copper, printed circuit board (PCB) material and electroniccomponents than what is used in a conventional RF coil. The RF coildisclosure herein includes parallel elongated wire conductors,encapsulated and separated by a dielectric material, forming the coilelement. The parallel wires form a low reactance structure without needfor discrete capacitors. The minimal conductor, sized to keep lossestolerable, eliminates much of the capacitance between coil loops, andreduces electric field coupling. By interfacing with a large samplingimpedance, currents are reduced and magnetic field coupling isminimized. The electronics are minimized in size and content to keepmass and weight low and prevent excessive interaction with the desiredfields. Packaging can now be extremely flexible which allows conformingto anatomy, optimizing signal to noise ratio (SNR) and imagingacceleration.

A traditional RF receive coil for MR is comprised of several conductiveintervals joined between themselves by capacitors. By adjusting thecapacitors' capacitances, the impedance of the RF coil may be brought toits minimal value, usually characterized by low resistance. At resonantfrequency, stored magnetic and electric energy alternate periodically.Each conductive interval, due to its length and width, possesses acertain self-capacitance, where electric energy is periodically storedas static electricity. The distribution of this electricity takes placeover the entire conductive interval length of the order of 5-15 cm,causing similar range electric dipole field. In a proximity of a largedielectric load, self-capacitance of the intervals change—hence detuningof the coil. In case of a lossy dielectric, dipole electric field causesJoule dissipation characterized by an increase overall resistanceobserved by the coil.

In contrast, the RF coil of the present disclosure represents almost anideal magnetic dipole antenna as its common mode current is uniform inphase and amplitude along its perimeter. The capacitance of the RF coilis built between at least two wires along the perimeter of the loop. Theconservative electric field is strictly confined within the smallcross-section of the parallel wires and dielectric filler material. Inthe case of two RF coil loops overlapping, the parasitic capacitance atthe cross-overs is greatly reduced in comparison to two overlappedcopper traces of traditional RF coils. RF coil thin cross-sectionsallows better magnetic decoupling and reduces or eliminates criticaloverlap between two loops in comparison to two traditional trace-basedcoil loops.

FIG. 1 illustrates a magnetic resonance imaging (MRI) apparatus 10 thatincludes a superconducting magnet unit 12, a gradient coil unit 13, anRF coil unit 14, an RF body or volume coil unit 15, a transmit/receive(T/R) switch 20, an RF driver unit 22, a gradient coil driver unit 23, adata acquisition unit 24, a controller unit 25, a patient table or bed26, a data processing unit 31, an operating console unit 32, and adisplay unit 33. In one example, the RF coil unit 14 is a surface coil,which is a local coil that is typically placed proximate to the anatomyof interest of a subject 16. Herein, the RF body coil unit 15 is atransmit coil that transmits RF signals, and the local surface RF coilunit 14 receives the MR signals. As such, the transmit body coil (e.g.,RF body coil unit 15) and the surface receive coil (e.g., RF coil unit14) are independent but electromagnetically coupled structures. The MRIapparatus 10 transmits electromagnetic pulse signals to the subject 16placed in an imaging space 18 with a static magnetic field formed toperform a scan for obtaining magnetic resonance signals from the subject16 to reconstruct an image of a slice of the subject 16 based on themagnetic resonance signals thus obtained by the scan.

The superconducting magnet unit 12 includes, for example, an annularsuperconducting magnet, which is mounted within a toroidal vacuumvessel. The magnet defines a cylindrical space surrounding the subject16, and generates a constant, strong, uniform, static magnetic fieldalong the Z direction of the cylindrical space.

The MRI apparatus 10 also includes the gradient coil unit 13 thatgenerates a gradient magnetic field in the imaging space 18 so as toprovide the magnetic resonance signals received by the RF coil unit 14with three-dimensional positional information. The gradient coil unit 13includes three gradient coil systems, each of which generates a gradientmagnetic field, which inclines into one of three spatial axesperpendicular to each other, and generates a gradient magnetic field ineach of frequency encoding direction, phase encoding direction, andslice selection direction in accordance with the imaging condition. Morespecifically, the gradient coil unit 13 applies a gradient magneticfield in the slice selection direction of the subject 16, to select theslice; and the RF body coil unit 15 transmits an RF signal to a selectedslice of the subject 16 and excites it. The gradient coil unit 13 alsoapplies a gradient magnetic field in the phase encoding direction of thesubject 16 to phase encode the magnetic resonance signals from the sliceexcited by the RF signal. The gradient coil unit 13 then applies agradient magnetic field in the frequency encoding direction of thesubject 16 to frequency encode the magnetic resonance signals from theslice excited by the RF signal.

The RF coil unit 14 is disposed, for example, to enclose the region tobe imaged of the subject 16. In some examples, the RF coil unit 14 maybe referred to as the surface coil or the receive coil. In the staticmagnetic field space or imaging space 18 where a static magnetic fieldis formed by the superconducting magnet unit 12, the RF coil unit 15transmits, based on a control signal from the controller unit 25, an RFpulse that is an electromagnet wave to the subject 16 and therebygenerates a high-frequency magnetic field. This excites a spin ofprotons in the slice to be imaged of the subject 16. The RF coil unit 14receives, as a magnetic resonance signal, the electromagnetic wavegenerated when the proton spin thus excited in the slice to be imaged ofthe subject 16 returns into alignment with the initial magnetizationvector. In some embodiments, the RF coil unit 14 may transmit the RFpulse and receive the MR signal. In other embodiments, the RF coil unit14 may only be used for receiving the MR signals, but not transmittingthe RF pulse.

The RF body coil unit 15 is disposed, for example, to enclose theimaging space 18, and produces RF magnetic field pulses orthogonal tothe main magnetic field produced by the superconducting magnet unit 12within the imaging space 18 to excite the nuclei. In contrast to the RFcoil unit 14, which may be disconnected from the MR apparatus 10 andreplaced with another RF coil unit, the RF body coil unit 15 is fixedlyattached and connected to the MRI apparatus 10. Furthermore, whereaslocal coils such as those comprising the RF coil unit 14 can transmit toor receive signals from only a localized region of the subject 16, theRF body coil unit 15 generally have a larger coverage area. The RF bodycoil unit 15 may be used to transmit or receive signals to the wholebody of the subject 16, for example. Using receive-only local coils andtransmit body coils provides a uniform RF excitation and good imageuniformity at the expense of high RF power deposited in the subject. Fora transmit-receive local coil, the local coil provides the RF excitationto the region of interest and receives the MR signal, thereby decreasingthe RF power deposited in the subject. It should be appreciated that theparticular use of the RF coil unit 14 and/or the RF body coil unit 15depends on the imaging application.

The T/R switch 20 can selectively electrically connect the RF body coilunit 15 to the data acquisition unit 24 when operating in receive mode,and to the RF driver unit 22 when operating in transmit mode. Similarly,the T/R switch 20 can selectively electrically connect the RF coil unit14 to the data acquisition unit 24 when the RF coil unit 14 operates inreceive mode, and to the RF driver unit 22 when operating in transmitmode. When the RF coil unit 14 and the RF body coil unit 15 are bothused in a single scan, for example if the RF coil unit 14 is configuredto receive MR signals and the RF body coil unit 15 is configured totransmit RF signals, then the T/R switch 20 may direct control signalsfrom the RF driver unit 22 to the RF body coil unit 15 while directingreceived MR signals from the RF coil unit 14 to the data acquisitionunit 24. The coils of the RF body coil unit 15 may be configured tooperate in a transmit-only mode, a receive-only mode, or atransmit-receive mode. The coils of the local RF coil unit 14 may beconfigured to operate in a transmit-receive mode or a receive-only mode.

The RF driver unit 22 includes a gate modulator (not shown), an RF poweramplifier (not shown), and an RF oscillator (not shown) that are used todrive the RF coil unit 15 and form a high-frequency magnetic field inthe imaging space 18. The RF driver unit 22 modulates, based on acontrol signal from the controller unit 25 and using the gate modulator,the RF signal received from the RF oscillator into a signal ofpredetermined timing having a predetermined envelope. The RF signalmodulated by the gate modulator is amplified by the RF power amplifierand then output to the RF coil unit 15.

The gradient coil driver unit 23 drives the gradient coil unit 13 basedon a control signal from the controller unit 25 and thereby generates agradient magnetic field in the imaging space 18. The gradient coildriver unit 23 includes three systems of driver circuits (not shown)corresponding to the three gradient coil systems included in thegradient coil unit 13.

The data acquisition unit 24 includes a pre-amplifier (not shown), aphase detector (not shown), and an analog/digital converter (not shown)used to acquire the magnetic resonance signals received by the RF coilunit 14. In the data acquisition unit 24, the phase detector phasedetects, using the output from the RF oscillator of the RF driver unit22 as a reference signal, the magnetic resonance signals received fromthe RF coil unit 14 and amplified by the pre-amplifier, and outputs thephase-detected analog magnetic resonance signals to the analog/digitalconverter for conversion into digital signals. The digital signals thusobtained are output to the data processing unit 31.

The MRI apparatus 10 includes a table 26 for placing the subject 16thereon. The subject 16 may be moved inside and outside the imagingspace 18 by moving the table 26 based on control signals from thecontroller unit 25.

The controller unit 25 includes a computer and a recording medium onwhich a program to be executed by the computer is recorded. The programwhen executed by the computer causes various parts of the apparatus tocarry out operations corresponding to pre-determined scanning. Therecording medium may comprise, for example, a ROM, flexible disk, harddisk, optical disk, magneto-optical disk, CD-ROM, or non-volatilememory. The controller unit 25 is connected to the operating consoleunit 32 and processes the operation signals input to the operatingconsole unit 32 and furthermore controls the table 26, RF driver unit22, gradient coil driver unit 23, and data acquisition unit 24 byoutputting control signals to them. The controller unit 25 alsocontrols, to obtain a desired image, the data processing unit 31 and thedisplay unit 33 based on operation signals received from the operatingconsole unit 32.

The operating console unit 32 includes user input devices such as atouchscreen, keyboard and a mouse. The operating console unit 32 is usedby an operator, for example, to input such data as an imaging protocoland to set a region where an imaging sequence is to be executed. Thedata about the imaging protocol and the imaging sequence executionregion are output to the controller unit 25.

The data processing unit 31 includes a computer and a recording mediumon which a program to be executed by the computer to performpredetermined data processing is recorded. The data processing unit 31is connected to the controller unit 25 and performs data processingbased on control signals received from the controller unit 25. The dataprocessing unit 31 is also connected to the data acquisition unit 24 andgenerates spectrum data by applying various image processing operationsto the magnetic resonance signals output from the data acquisition unit24.

The display unit 33 includes a display device and displays an image onthe display screen of the display device based on control signalsreceived from the controller unit 25. The display unit 33 displays, forexample, an image regarding an input item about which the operatorinputs operation data from the operating console unit 32. The displayunit 33 also displays a two-dimensional (2D) slice image orthree-dimensional (3D) image of the subject 16 generated by the dataprocessing unit 31.

During a scan, RF coil array interfacing cables (not shown) may be usedto transmit signals between the RF coils (e.g., RF coil unit 14 and RFbody coil unit 15) and other aspects of the processing system (e.g.,data acquisition unit 24, controller unit 25, and so on), for example tocontrol the RF coils and/or to receive information from the RF coils. Asexplained previously, the RF body coil unit 15 is a transmit coil thattransmits RF signals, and the local surface RF coil 14 receives the MRsignals. More generally, RF coils are used to transmit RF excitationsignals (“transmit coil”), and to receive the MR signals emitted by animaging subject (“receive coil”). In an example, the transmit andreceive coils are a single mechanical and electrical structure or arrayof structures, with transmit/receive mode switchable by auxiliarycircuitry. In other examples, the transmit body coil (e.g., RF body coilunit 15) and the surface receive coil (e.g., RF coil unit 14) may beindependent structures that are physically coupled to each other via adata acquisition unit or other processing unit. For enhanced imagequality, however, it may be desirable to provide a receive coil that ismechanically and electrically isolated from the transmit coil. In suchcase it is desirable that the receive coil, in its receive mode, beelectromagnetically coupled to and resonant with an RF “echo” that isstimulated by the transmit coil. However, during transmit mode, it maybe desirable that the receive coil is electromagnetically decoupled fromand therefore not resonant with the transmit coil, during actualtransmission of the RF signal. Such decoupling averts a potentialproblem of noise produced within the auxiliary circuitry when thereceive coil couples to the full power of the RF signal. Additionaldetails regarding the uncoupling of the receive RF coil will bedescribed below.

As mentioned previously, traditional RF coils may include acid etchedcopper traces (loops) on PCBs with lumped electronic components (e.g.,capacitors, inductors, baluns, resistors, etc.), matching circuitry,decoupling circuitry, and pre-amplifiers. Such a configuration istypically very bulky, heavy and rigid, and requires relatively strictplacement of the coils relative to each other in an array to preventcoupling interactions among coil elements that may degrade imagequality. As such, traditional RF coils and RF coil arrays lackflexibility and hence may not conform to patient anatomy, degradingimaging quality and patient comfort.

Thus, according to embodiments disclosed herein, an RF coil array, suchas RF coil unit 14, may include distributed capacitance wires ratherthan copper traces on PCBs with lumped electronic components. As aresult, the RF coil array may be lightweight and flexible, allowingplacement in low-cost, lightweight, waterproof, and/or flame retardantfabrics or materials. The coupling electronics portion coupling the loopportion of the RF coil (e.g., the distributed capacitance wire) may beminiaturized and utilize a low input impedance pre-amplifier, which isoptimized for high source impedance (e.g., due to impedance matchingcircuitry) and allows flexible overlaps among coil elements in an RFcoil array. Further, the RF coil array interfacing cable between the RFcoil array and system processing components may be flexible and includeintegrated transparency functionality in the form of distributed baluns,which allows rigid electronic components to be avoided and aids inspreading of the heat load.

Turning now to FIG. 2, a schematic view of an RF coil 202 including aloop portion 201 coupled to a controller unit 210 via couplingelectronics portion 203 and a coil-interfacing cable 212 is shown. Inone example, the RF coil may be a surface receive coil, which may besingle- or multi-channel. The RF coil 202 is one non-limiting example ofRF coil unit 14 of FIG. 1 and as such may operate at one or morefrequencies in the MRI apparatus 10. The coil-interfacing cable 212 maybe a coil-interfacing cable extending between the electronics portion203 and an interfacing connector of an RF coil array or a RF coil arrayinterfacing cable extending between the interfacing connector of the RFcoil array and the MRI system controller unit 210. The controller unit210 may be associated with and/or may be a non-limiting example of thedata processing unit 31 or controller unit 25 in FIG. 1.

The coupling electronics portion 203 may be coupled to the loop portion201 of the RF coil 202. Herein, the coupling electronics portion 203 mayinclude a decoupling circuit 204, impedance inverter circuit 206, and apre-amplifier 208. The decoupling circuit 204 may effectively decouplethe RF coil during a transmit operation. Typically, the RF coil 202 inits receive mode may be coupled to a body of a subject being imaged bythe MR apparatus in order to receive echoes of the RF signal transmittedduring the transmit mode. If the RF coil 202 is not used fortransmission, then it may be necessary to decouple the RF coil 202 fromthe RF body coil while the RF body coil is transmitting the RF signal.The decoupling of the receive coil from the transmit coil may beachieved using resonance circuits and PIN diodes, microelectromechanicalsystems (MEMS) switches, or another type of switching circuitry. Herein,the switching circuitry may activate detuning circuits operativelyconnected to the RF coil 202.

The impedance inverter circuit 206 may form an impedance matchingnetwork between the RF coil 202 and the pre-amplifier 208. The impedanceinverter circuit 206 is configured to transform a coil impedance of theRF coil 202 into an optimal source impedance for the pre-amplifier 208.The impedance inverter circuit 206 may include an impedance matchingnetwork and an input balun. The pre-amplifier 208 receives MR signalsfrom the corresponding RF coil 202 and amplifies the received MRsignals. In one example, the pre-amplifier may have a low inputimpedance that is configured to accommodate a relatively high blockingor source impedance. Additional details regarding the RF coil andassociated coupling electronics portion will be explained in more detailbelow with respect to FIGS. 3-5.

The coupling electronics portion 203 may be packaged in a very small PCBapproximately 2 cm² in size or smaller. The PCB may be protected with aconformal coating or an encapsulating resin.

The coil-interfacing cable 212, such as a RF coil array interfacingcable, may be used to transmit signals between the RF coils and otheraspects of the processing system, for example to control the RF coilsand/or to receive information from the RF coils. The RF coil arrayinterfacing cables may be disposed within the bore or imaging space ofthe MRI apparatus (such as MRI apparatus 10 of FIG. 1) and subjected toelectro-magnetic fields produced and used by the MRI apparatus. In MRIsystems, coil-interfacing cables, such as coil-interfacing cable 212,may support transmitter-driven common-mode currents, which may in turncreate field distortions and/or unpredictable heating of components.Typically, common-mode currents are blocked by using baluns. Baluns orcommon-mode traps provide high common-mode impedances, which in turnreduces the effect of transmitter-driven currents.

Thus, coil-interfacing cable 212 may include one or more baluns. Intraditional coil-interfacing cables, baluns are positioned with arelatively high density, as high dissipation/voltages may develop if thebalun density is too low or if baluns are positioned at an inappropriatelocation. However, this dense placement may adversely affectflexibility, cost, and performance. As such, the one or more baluns inthe coil-interfacing cable may be continuous baluns to ensure no highcurrents or standing waves, independent of positioning. The continuousbaluns may be distributed, flutter, and/or butterfly baluns. Additionaldetails regarding the coil-interfacing cable and baluns will bepresented below with respect to FIGS. 15-17.

FIG. 3 is a schematic of an RF coil 301 having segmented conductorsformed in accordance with an embodiment. RF coil 301 is a non-limitingexample of RF coil 202 of FIG. 2 and as such includes loop portion 201and coupling electronics portion 203 of RF coil 202. The couplingelectronics portion allows the RF coil to transmit and/or receive RFsignals when driven by the data acquisition unit 124 (shown in FIG. 1).In the illustrated embodiment, the RF coil 301 includes a firstconductor wire 300 and a second conductor wire 302. The first and secondconductor wires 300, 302 may be segmented such that the conductors forman open circuit (e.g., form a monopole). The segments of the conductorwires 300, 302 may have different lengths, as is discussed below. Thelength of the first and second conductor wires 300, 302 may be varied toachieve a desired distributed capacitance, and accordingly, a desiredresonance frequency.

The first conductor wire 300 includes a first segment 304 and a secondsegment 306. The first segment 304 includes a driven end 312 at aninterface terminating to coupling electronics portion 203, which will bedescribed in more detail below. The first segment 304 also includes afloating end 314 that is detached from a reference ground, therebymaintaining a floating state. The second segment 306 includes a drivenend 316 at the interface terminating to the coupling electronics portionand a floating end 318 that is detached from a reference ground.

The second conductor wire 302 includes a first segment 308 and a secondsegment 310. The first segment 308 includes a driven end 320 at theinterface. The first segment 308 also includes a floating end 322 thatis detached from a reference ground, thereby maintaining a floatingstate. The second segment 310 includes a driven end 324 at theinterface, and a floating end 326 that is detached from a referenceground. The driven end 324 may terminate at the interface such that end324 is only coupled to the first conductor through the distributedcapacitance. The capacitors shown around the loop between the conductorsrepresent the capacitance between the wires.

The first conductor wire 300 and the second conductor wire 302 exhibit adistributed capacitance that grows along the length of the first andsecond segments 304, 306, 308, 310. The first segments 304, 308 may havea different length than the second segments 306, 310. The relativedifference in length between the first segments 304, 308 and the secondsegments 306, 310 may be used to produce an effective LC circuit havinga resonance frequency at the desired center frequency. For example, byvarying the length of the first segments 304, 308 relative to thelengths of the second segments 306, 310, an integrated distributedcapacitance may be varied.

In the illustrated embodiment, the first and second conductor wires 300,302 are shaped into a loop portion that terminates to an interface. Butin other embodiments, other shapes are possible. For example, the loopportion may be a polygon, shaped to conform the contours of a surface(e.g., housing), and/or the like. The loop portion defines a conductivepathway along the first and second conductor wires. The first and secondconductor wires are void of any discrete or lumped capacitive orinductive elements along an entire length of the conductive pathway. Theloop portion may also include loops of varying gauge of stranded orsolid conductor wire, loops of varying diameters with varying lengths ofthe first and second conductor wires 300, 302, and/or loops of varyingspacing between the first and second conductor wires. For example, eachof the first and second conductors may have no cuts or gaps (nosegmented conductors) or one or more cuts or gaps (segmented conductors)at various locations along the conductive pathway.

Distributed capacitance (DCAP), as used herein and also referred to asintegrated capacitance, represents a capacitance exhibited betweenconductors along the length of the conductors and is void of discrete orlumped capacitive components and discrete or lumped inductivecomponents. In the examples herein, the capacitance may grow in an evenand uniform manner along the length of the first and second conductorwires 300, 302.

A dielectric material 303 encapsulates and separates the first andsecond conductor wires 300, 302. The dielectric material 303 may beselectively chosen to achieve a desired distributive capacitance. Thedielectric material 303 may be based on a desired permittivity E to varythe effective capacitance of the loop portion. For example, thedielectric material 303 may be air, rubber, plastic, or any otherdielectric material. In one example, the dielectric material may bepolytetrafluoroethylene (pTFE). For example, the dielectric material 303may be an insulating material surrounding the parallel conductiveelements of the first and second conductor wires 300, 302.Alternatively, the first and second conductor wires 300, 302 may betwisted upon one another to form a twisted pair cable. As anotherexample, the dielectric material 303 may be a plastic material. Thefirst and second conductor wires 300, 302 may form a coaxial structurein which the plastic dielectric material 303 separates the first andsecond conductor wires. As another example, the first and secondconductor wires may be configured as planar strips.

The coupling electronics portion 203 is operably and communicativelycoupled to the RF driver unit 22, the data acquisition unit 24,controller unit 25, and/or data processing unit 31 to allow the RF coil102 to transmit and/or receive RF signals. In the illustratedembodiment, the coupling electronics portion 203 includes a signalinterface 358 configured to transmit and receive the RF signals. Thesignal interface 358 may transmit and receive the RF signals via acable. The cable may be a 3-conductor triaxial cable having a centerconductor, an inner shield, and an outer shield. The center conductor isconnected to the RF signal and pre-amp control (RF), the inner shield isconnected to ground (GND), and the outer shield is connected to themulti-control bias (diode decoupling control) (MC_BIAS). A 10V powerconnection may be carried on the same conductor as the RF signal.

As explained above with respect to FIG. 2, the coupling electronicsportion 203 includes a decoupling circuit, impedance inverter circuit,and pre-amplifier. As illustrated in FIG. 3, the decoupling circuitincludes a decoupling diode 360. The decoupling diode 360 may beprovided with voltage from MC_BIAS, for example, in order to turndecoupling diode 360 on. When on, decoupling diode 360 causes conductor300 to short with conductor 302, thus causing the coil to beoff-resonance and hence decouple the coil during a transmit operation,for example.

It should be understood that the decoupling circuit shown in FIG. 3 isfor illustration not for limitation. Any appropriate decouplingconfiguration can be used to decouple the RF coil during the transmitoperation.

The impedance inverter circuit includes a plurality of inductors,including first inductor 370 a, second inductor 370 b, and thirdinductor 370 c; a plurality of capacitors, including first capacitor 372a, a second capacitor 372 b, a third capacitor 372 c, and a fourthcapacitor 372 d; and a diode 374. The impedance inverter circuitincludes matching circuitry and an input balun. As shown, the inputbalun is a lattice balun that comprises first inductor 370 a, secondinductor 370 b, first capacitor 372 a, and second capacitor 372 b. Inone example, diode 374 limits the direction of current flow to block RFreceive signals from proceeding into decoupling bias branch (MC_BIAS).

The pre-amplifier 362 may be a low input impedance pre-amplifier that isoptimized for high source impedance by the impedance matching circuitry.The pre-amplifier may have a low noise reflection coefficient, γ, and alow noise resistance, Rn. In one example, the pre-amplifier may have asource reflection coefficient of γ substantially equal to 0.0 and anormalized noise resistance of Rn substantially equal to 0.0 in additionto the low noise figure. However, γ values substantially equal to orless than 0.1 and Rn values substantially equal to or less than 0.2 arealso contemplated. With the pre-amplifier having the appropriate γ andRn values, the pre-amplifier provides a blocking impedance for RF coil301 while also providing a large noise circle in the context of a SmithChart. As such, current in RF coil 301 is minimized, the pre-amplifieris effectively noise matched with RF coil 301 output impedance. Having alarge noise circle, the pre-amplifier yields an effective SNR over avariety of RF coil impedances while producing a high blocking impedanceto RF coil 301.

In some examples, the pre-amplifier 362 may include an impedancetransformer that includes a capacitor and an inductor. The impedancetransformer may be configured to alter the impedance of thepre-amplifier to effectively cancel out a reactance of thepre-amplifier, such as capacitance caused by a parasitic capacitanceeffect. Parasitic capacitance effects can be caused by, for example, aPCB layout of the pre-amplifier or by a gate of the pre-amplifier.Further, such reactance can often increase as the frequency increases.Advantageously, however, configuring the impedance transformer of thepre-amplifier to cancel, or at least minimize, reactance maintains ahigh impedance (i.e. a blocking impedance) to RF coil 301 and aneffective SNR without having a substantial impact on the noise figure ofthe pre-amplifier. The lattice balun described above may be anon-limiting example of an impedance transformer.

In examples, the pre-amplifier described herein may be a low inputimpedance pre-amplifier. For example, in some embodiments, a “relativelylow” input impedance of the preamplifier is less than approximately 5ohms at resonance frequency. The coil impedance of the RF coil 301 mayhave any value, which may be dependent on coil loading, coil size, fieldstrength, and/or the like. Examples of the coil impedance of the RF coil301 include, but are not limited to, between approximately 2 ohms andapproximately 10 ohms at 1.5 T magnetic field strength, and/or the like.The impedance inverter circuitry is configured to transform the coilimpedance of the RF coil 301 into a relatively high source impedance.For example, in some embodiments, a “relatively high” source impedanceis at least approximately 100 ohms and may be greater than 150 ohms.

The impedance transformer may also provide a blocking impedance to theRF coil 301. Transformation of the coil impedance of the RF coil 301 toa relative high source impedance may enable the impedance transformer toprovide a higher blocking impedance to the RF coil 301. Exemplary valuesfor such higher blocking impedances include, for example, a blockingimpedance of at least 500 ohms, and at least 1000 ohms.

FIG. 4 is a schematic of an RF coil 401 and coupling electronics portion203 according to another embodiment. The RF coil of FIG. 4 is anon-limiting example of the RF coil and coupling electronics of FIG. 2,and as such includes a loop portion 201 and coupling electronics portion203. The coupling electronics allows the RF coil to transmit and/orreceive RF signals when driven by the data acquisition unit 124 (shownin FIG. 1). The RF coil 401 includes a first conductor wire 400 inparallel with a second conductor wire 402. At least one of the first andsecond conductor wires 400, 402 are elongated and continuous.

In the illustrated embodiment, the first and second conductor wires 400,402 are shaped into a loop portion that terminates to an interface. Butin other embodiments, other shapes are possible. For example, the loopportion may be a polygon, shaped to conform the contours of a surface(e.g., housing), and/or the like. The loop portion defines a conductivepathway along the first and second conductor wires 400, 402. The firstand second conductor wires 400, 402 are void of any discrete or lumpedcapacitive or inductive components along an entire length of theconductive pathway. The first and second conductor wires 400, 402 areuninterrupted and continuous along an entire length of the loop portion.The loop portion may also include loops of varying gauge of stranded orsolid conductor wire, loops of varying diameters with varying lengths ofthe first and second conductor wires 400, 402, and/or loops of varyingspacing between the first and second conductors. For example, each ofthe first and second conductors may have no cuts or gaps (no segmentedconductors) or one or more cuts or gaps (segmented conductors) atvarious locations along the conductive pathway.

The first and second conductor wires 400, 402 have a distributedcapacitance along the length of the loop portion (e.g., along the lengthof the first and second conductor wires 400, 402). The first and secondconductor wires 400, 402 exhibit a substantially even and uniformcapacitance along the entire length of the loop portion. Distributedcapacitance (DCAP), as used herein, represents a capacitance exhibitedbetween conductors along the length of the conductors and is void ofdiscrete or lumped capacitive components and discrete or lumpedinductive components. In the examples herein, the capacitance may growin an even and uniform manner along the length of the first and secondconductor wires 400, 402. At least one of the first and second conductorwires 400, 402 are elongated and continuous. In the illustratedembodiment, both the first and second conductor wires 400, 402 areelongated and continuous. But in other embodiments, only one of thefirst or second conductor wires 400, 402 may be elongated andcontinuous. The first and second conductor wires 400, 402 formcontinuous distributed capacitors. The capacitance grows at asubstantially constant rate along the length of the conductor wires 400,402. In the illustrated embodiment, the first and second conductor wires400, 402 form elongated continuous conductors that exhibits DCAP alongthe length of the first and second conductor wires 400, 402. The firstand second conductor wires 400, 402 are void of any discrete capacitiveand inductive components along the entire length of the continuousconductors between terminating ends of the first and second conductorwires 400, 402. For example, the first and second conductor wires 400,402 do not include any discrete capacitors, nor any inductors along thelength of the loop portion.

A dielectric material 403 separates the first and second conductor wires400, 402. The dielectric material 403 may be selectively chosen toachieve a desired distributive capacitance. The dielectric material 403may be based on a desired permittivity E to vary the effectivecapacitance of the loop portion. For example, the dielectric material403 may be air, rubber, plastic, or any other dielectric material. Inone example, the dielectric material may be polytetrafluoroethylene(pTFB). For example, the dielectric material 403 may be an insulatingmaterial surrounding the parallel conductive elements of the first andsecond conductor wires 400, 402. Alternatively, the first and secondconductor wires 400, 402 may be twisted upon one another to from atwisted pair cable. As another example, the dielectric material 403 maybe a plastic material. The first and second conductor wires 400, 402 mayform a coaxial structure in which the plastic dielectric material 403separates the first and second conductor wires 400, 402. As anotherexample, the first and second conductor wires 400, 402 may be configuredas planar strips.

The first conductor wire 400 includes a first terminating end 412 and asecond terminating end 416 that terminates at the interface. The firstterminating end 412 is coupled to the coupling electronics portion 203.The first terminating end 412 may also be referred to herein as a “driveend.” The second terminating end 416 is also referred to herein as a“second drive end.”

The second conductor 402 includes a first terminating end 420 and asecond terminating end 424 that terminates at the interface. The firstterminating end 420 is coupled to the coupling electronics portion 203.The first terminating end 420 may also be referred to herein as a “driveend.” The second terminating end 424 is also referred to herein as a“second drive end.”

The loop portion 201 of the RF coil 401 is coupled to couplingelectronics portion 203. The coupling electronics portion 203 may be thesame coupling electronics described above with respect to FIGS. 2 and 3,and hence like reference numbers are given to like components andfurther description is dispensed with.

As appreciated by FIGS. 3 and 4, the two parallel conductor wiresforming the loop portion of an RF coil may each be continuous, asillustrated in FIG. 4, or one or both of the conductors may benon-continuous, as illustrated in FIG. 3. For example, both conductorwires shown in FIG. 3 may include cuts, resulting in each conductorbeing comprised of two segments. The resulting space between conductorsegments may be filled with the dielectric material that encapsulatesand surrounds the conductors. The two cuts may be positioned atdifferent locations, e.g., one cut at 135° and the other cut at 225°(relative to where the loop portion interfaces with the couplingelectronics). By including discontinuous segments, the resonancefrequency of the coil may be adjusted. For example, for an RF coil thatincludes two continuous parallel conductor wires encapsulated andseparated by a dielectric, the resonance frequency may be a smaller,first resonance frequency. If the RF coil instead includes onediscontinuous conductor wire (e.g., where one of the conductors is cutand filled with the dielectric material) and one continuous conductorwire, with all other parameters (e.g., conductor diameter, loopdiameter, dielectric material) being the same, the resonance frequencyof the RF coil may be a larger, second resonance frequency. In this way,parameters of the loop portion, including conductor wire gauge, loopdiameter, spacing between conductors, dielectric material selectionand/or thickness, and conductor segment number and lengths, may beadjusted to tune the RF coil to a desired resonance frequency.

The RF coils described herein (e.g., the RF coils described above withrespect to FIGS. 2-4) generate distributed (also referred to asincorporated) capacitance between two conductor wires (e.g., between thetwo looped wires). The distributed capacitance results from theapproximately linear varying currents in the two conductor wires: in oneconductor wire, the current linearly ascends and in the other conductorwire, current linearly descends. From the continuity equation(∇·j+∂_(t)ρ=0), current linearly varying along its path leads toconstant positive charge distribution. Linearly descending current leadsto constant negative charge distribution. Referring to FIG. 4 as anexample, current may descend in conductor wire 400 and may ascend inconductor wire 402. In this way, the loop portion of the RF coilsdescribed herein may act as a folded electric dipole antenna, wherecommon mode current is approximately constant and the current on thedifferent wires (e.g., non-common mode current) varies along thecircumference of the wires.

FIG. 5 shows a cross-sectional view 450 of loop portion 201 of RF coil401 taken across line A-A′ of FIG. 4. As appreciated by FIG. 5, loopportion 201 of RF coil 401 includes first conductor wire 400 and secondconductor wire 402 surrounded by and encapsulated in dielectric material403. Each conductor may have a suitable cross-sectional shape, herein acircular cross-sectional shape having a radius r. However, othercross-sectional shapes for the conductors are possible, such asrectangular, triangular, hexagonal, etc. The conductors may be separatedby a suitable distance d (where d is the distance between the centers ofthe conductors). In one example, the radius r of each conductor may be0.15 mm and the distance d between conductors may be 0.5 mm, but otherdimensions are possible and may be selected to achieve a desiredcapacitance. As shown, first conductor wire 400 flows current in adescending manner, represented by the minus symbol and also referred toherein as descending current. Second conductor wire 402 flows current inan ascending manner, represented by the plus symbol and also referred toherein as ascending current.

As explained above, by adjusting one or more of the distance d betweenthe conductors, diameter of the conductors, number of cuts to theconductors, and the properties of the dielectric material, thecapacitance of the loop portion may be adjusted. However, for some MRIapplications, an RF coil comprising only two conductors may not providesufficient capacitance. For example, imaging of carbon 14 may beperformed at a low resonance frequency of 32 megahertz (MHz), whichrequires larger capacitance than what two conductor wires can provide.Another mechanism by which RF coil loop capacitance may be adjustedincludes adjusting the number of conductors included in the loopportion. FIGS. 6-9, described in more detail below, illustrate exampleRF coil configurations that include more than two conductor wires.

FIG. 6 shows a loop portion 601 of an RF coil including four parallelconductor wires. FIG. 7 shows a cross-sectional view 700 of the loopportion 601 of the RF coil of FIG. 6. Loop portion 601 includes fourconductor wires including first conductor wire 602, second conductorwire 604, third conductor wire 606, and fourth conductor wire 608, allencapsulated in a common dielectric material 710 (illustrated in FIG.7). Similar to the RF coils described above, each conductor wire may bea conductive wire or strip, such as a copper wire, and the dielectricmaterial may be pTFE, rubber, or other suitable dielectric material.FIG. 6 shows the four conductor wires somewhat schematically (e.g.,flattened and/or on the same plane) for visual purposes, but it is to beunderstood that the actual planar arrangement of the conductors maydiffer from that shown in FIG. 6, such as the arrangement shown in FIG.7 and described in more detail below.

The RF coil illustrated in FIGS. 6 and 7 may include some similarfeatures as the RF coils described above with respect to FIGS. 2-5,including the loop portion 601 being coupled to coupling electronics atan interface. To facilitate the coupling to the coupling electronics,the first conductor 602 includes a first terminating end 612, similar tothe first terminating end 412 described above with respect to FIG. 4.The first conductor 602 is electrically coupled to the third conductor606 at a second terminating end 616 that terminates at the interface,similar to the second terminating end 416 of FIG. 4. The firstterminating end 612 is coupled to the coupling electronics. The firstterminating end 612 may also be referred to herein as a “drive end.” Thesecond terminating end 616 is also referred to herein as a “second driveend.”

The second conductor 604 may be electrically coupled to the fourthconductor 608 at a first terminating end 620 that is coupled to thecoupling electronics, similar to the first terminating end 420 of FIG.4. Fourth conductor 608 includes a second terminating end 624 thatterminates at the interface, similar to the second terminating end 424of FIG. 4. The first terminating end 620 may also be referred to hereinas a “drive end.” The second terminating end 624 is also referred toherein as a “second drive end.” In this way, the loop portion 601 iscoupled to coupling electronics, and the coupling electronics may be thesame coupling electronics described above with respect to FIGS. 2 and 3.The second conductor 604 and the third conductor 606 may each have afloating end embedded in the loop portion.

The four conductors may be arranged into a pair of vertical conductors(including first conductor 602 and third conductor 606) and a pair ofhorizontal conductors (including second conductor 604 and fourthconductor 608). The vertical conductors, as shown, may flow ascendingcurrent while the horizontal conductors may flow descending current. Byincluding four conductor wires arranged into two parallel pairs, thecapacitance of the loop portion 601 may be increased relative to a loopthat includes only two conductors. For example, the conductor wiresshown in FIG. 7 may have the same radius r and be comprised of the samematerial as the conductors illustrated in FIG. 5 (e.g., r of 0.15 mm)and may be spaced apart by the same distance d (at least adjacentconductors may be spaced apart by the distance d, while non-adjacentconductors may be spaced apart by a larger distance), such as 0.5 mm, ofthe same dielectric material as the loop portion illustrated in FIG. 5.The additional conductor wires may increase the capacitance of the loopportion, for example from approximately 50 pF to approximately 140 pF.

A further mechanism by which the capacitance of a loop portion of an RFcoil may be adjusted includes the type of dielectric material comprisingthe core of the loop portion. Referring to FIG. 7 as an example, loopportion 601 includes a core 720, around which the four conductors arearranged. Core 720 may be a hollowed out tube in the dielectric material710, traversing along the loop portion. Core 720 may be maintainedhollow (e.g., filled with air) in one example. In other examples, core720 may be filled with material having the same as or differentdielectric properties than the material comprising dielectric material710, such as pTFE, water, ceramic, and so forth. As the relativepermittivity of the core material increases, the capacitance of the loopportion also increases. For example, if the core is left follow (e.g.,filled with air), the loop portion may have a lower, first capacitance.If the core is filled with pTFE, the loop portion may have a higher,second capacitance. If the core is filled with water, the loop portionmay have an even higher, third capacitance.

While FIGS. 6 and 7 show an RF coil loop portion including fourconductor wires, more or fewer conductors are possible, such as threeconductors. An even number of interleaved wires assures uniformdistribution of the charge along the loop path and good confinement ofthe electric field within the multicore wire. An odd number of wires inprinciple is possible, but may be prone to wavelength effects—nonuniformcharge distribution along the loop path.

FIG. 8 shows a loop portion 801 of an RF coil including six parallelconductor wires. FIG. 9 shows a cross-sectional view 900 of the loopportion 801 of the RF coil of FIG. 8. Loop portion 801 includes sixconductor wires including first conductor wire 802, second conductorwire 804, third conductor wire 806, fourth conductor wire 808, fifthconductor wire 810, and sixth conductor wire 811, all encapsulated in acommon dielectric material 910 (illustrated in FIG. 9). Similar to theRF coils described above, each conductor may be a conductive wire orstrip, such as a copper wire, and the dielectric material may be pTFE,rubber, or other suitable dielectric material. FIG. 8 shows the sixconductors somewhat schematically (e.g., flattened and/or on the sameplane) for visual purposes, but it is to be understood that the actualplanar arrangement of the conductors may differ from that shown in FIG.8, such as the arrangement shown in FIG. 9 and described in more detailbelow.

The RF coil illustrated in FIGS. 8 and 9 may include some similarfeatures as the RF coils described above with respect to FIGS. 2-5,including the loop portion 801 being coupled to coupling electronics atan interface. To facilitate the coupling to the coupling electronics,the first conductor 802 wire is electrically coupled to the thirdconductor wire 806 and to the fifth conductor wire 810 at a firstterminating end 820 that terminates at the interface, similar to thefirst terminating end 420 of FIG. 4. The first terminating end 820 iscoupled to the coupling electronics. The first conductor wire 802 alsoincludes a second terminating end 824 that terminates at the interface,similar to the second terminating end 424 of FIG. 4.

The second conductor wire 804 may be electrically coupled to the fourthconductor wire 808 and the sixth conductor wire 811 at a firstterminating end 816 that is coupled to the coupling electronics, similarto the first terminating end 416 of FIG. 4. Sixth conductor wire 811includes a second terminating end 812 that terminates at the interface,similar to the second terminating end 412 of FIG. 4. In this way, theloop portion 801 is coupled to coupling electronics, and the couplingelectronics may be the same coupling electronics described above withrespect to FIGS. 2 and 3. The second conductor wire 804, the thirdconductor wire 806, the fourth conductor wire 808, and the fifthconductor wire 810 may each have a floating end embedded in the loopportion.

As shown in FIG. 9, the loop portion 801 includes the six conductorwires arranged in a hexagonal manner. The loop portion may be configured(e.g., coupled to the coupling electronics in such a manner) to flowascending and descending current in an every-other configuration. Forexample, first conductor wire 802, third conductor wire 806, and fifthconductor wire 810 may each flow ascending current, while secondconductor wire 804, fourth conductor wire 808, and sixth conductor wire811 may each flow descending current. Each conductor wire may have thesame radius r (such as 0.15 mm, similar to the conductors describedabove with respect to FIG. 5 and FIG. 6) and be spaced apart by the samedistance d (such as 0.5 mm as described above). The conductor wires mayeach be comprised of copper wire, for example, and the dielectricmaterial 910 may be pTFE, rubber, or the like. Due to the presence ofsix conductor wires, the loop portion 801 may have a higher capacitancethan loop portion 601 of FIG. 6. In one example, the capacitance of loopportion 801 may be approximately 220 pF.

Further, loop portion 801 may include a core 920, around which the sixconductor wires are arranged. Core 920 may be a hollowed out tube in thedielectric material 910, traversing along the loop portion. Core 920 maybe maintained hollow (e.g., filled with air) in one example. In otherexamples, core 920 may be filled with material having the same ordifferent dielectric properties that the material comprising dielectricmaterial 910, such as pTFE, water, ceramic, and so forth. As therelative permittivity of the core material increases, the capacitance ofthe loop portion also increases. For example, if the core is left follow(e.g., filled with air), the loop portion may have a lower, firstcapacitance. If the core is filled with pTFE, the loop portion may havea higher, second capacitance. If the core is filled with water, the loopportion may have an even higher, third capacitance. Further, the core920 (as well as core 720 of FIG. 7) may have a diameter that is selectedto achieve a desired capacitance. In an example, the core may have amaximum diameter that is a function of wire spacing and wire diameter.For example, the maximum diameter of the core may be twice the distancebetween the wires (the distance d of FIG. 9) minus the twice the radiusof the wires (the radius r of FIG. 9). In the examples presented abovewhere the distance is 0.5 mm and the radius is 0.15 mm, the core mayhave a maximum diameter of 0.7 mm.

In this way, by varying the number of conductive wires, wire diameter,number of cuts in the wire, dielectric material, and/or otherparameters, a broad range of distributed capacitance per unit length ofthe loop portion of the RF coil may be achieved, which may allow the RFcoil to be operated at a resonance frequency suited to a particular MRIenvironment (e.g., 1.5 T versus 3 T). As explained above, by increasingthe number of conductive wires, the capacitance may be increased. Thecapacitance may additionally or alternatively be varied by modifying thedielectric material core within the RF coil loop portion cross-section.Further, in some examples, to modify the capacitance of the RF coil loopportion, one or more cuts may be present on one or more of theconductors. When the RF coil loop portion includes more than twoconductors (e.g., four or six parallel wires as described above withrespect to FIGS. 6 and 8), the cuts to the conductors may be made onlyon the conductors flowing ascending current and not on the conductorsflowing descending current, or vice versa.

FIG. 10 is a graph 1000 illustrating conductor current as a function ofconductor length (in the example shown, the conductor circumference forlooped conductors), for conductors with descending current (plot 1002)and conductors with ascending current (plot 1004). As appreciated byFIG. 10, the current may descend linearly along a length of oneconductor wire (shown by plot 1002) and ascend linearly (in an amountproportional to the amount of descending current) along the length ofthe other conductor wire (shown by plot 1004).

FIGS. 11-13 illustrate various additional RF coil loop portionconfigurations. Each coil loop portion illustrated in FIGS. 11-13 is across-section of a loop portion, similar to the cross-sectional viewsshown in FIGS. 5, 7, and 9. Further, each loop portion illustrated inFIGS. 11-13 may be coupled to a respective coupling electronics portionin a manner similar to that described above with respect to FIGS. 3 and4.

FIG. 11 shows an example coaxial coil loop portion 1100 that includes around center conductor wire 1102, an outer concentric shield 1106, and adielectric material 1104 in between. The center conductor wire 1102 maybe similar to the conductor wires described above with respect to FIGS.2-4 (e.g., comprised of copper wire) and the dielectric material 1104may be similar to the dielectric materials described above (e.g.,comprised of pTFE). The outer concentric shield 1106 may encase orotherwise surround the dielectric material 1104 and center conductorwire 1102 and may be comprised of braided copper or other suitableconductive material. The center conductor, dielectric material, andouter shield all share a common central axis. Further, while not shownin FIG. 11, in some examples an outer jacket (e.g., comprised ofdielectric material) may surround the outer shield 1106. While twocoaxial conductors (center conductor wire 1102 and outer shield 1106)are shown in FIG. 11, an RF coil loop portion may include three or morecoaxial conductors, encapsulated and separated from each other bydielectric material.

FIG. 12 shows another example coaxial loop portion 1200 that includes aminner shield 1202, an outer concentric shield 1206, and a dielectricmaterial 1204 in between. The dielectric material may be similar to thedielectric materials described above (e.g., comprised of pTFE). Theinner shield 1202 and outer concentric shield 1206 may be comprised ofbraided copper or other suitable conductive material. The outerconcentric shield 1206 may encase or otherwise surround the dielectricmaterial and inner shield. The inner shield 1202 may surround a hollowcore, or the inner shield 1202 may surround a core comprising adielectric material. The inner shield, dielectric material, and outershield all share a common central axis. Further, while not shown in FIG.12, in some examples an outer jacket (e.g., comprised of dielectricmaterial) may surround the outer shield 1206. While two coaxialconductors (inner shield 1202 and outer shield 1206) are shown in FIG.12, an RF coil loop portion may include three or more coaxial shields,encapsulated and separated from each other by dielectric material.

FIG. 13 shows an example coil loop portion 1300 comprised of micro-stripconductors, including a first micro-strip conductor 1302 and a secondmicro-strip conductor 1304 separated by (and encased in) a dielectricmaterial 1303. The dielectric material may be similar to the dielectricmaterials described above (e.g., comprised of pTFE). The micro-stripconductors may be rectangular (in cross-section) parallel conductorsthat may be comprised of copper and may be looped in a manner similar tothe wire loops described above with respect to FIGS. 2-4. Further, whiletwo micro-strip conductors are shown in FIG. 13, an RF coil loop portionmay include more than two micro-strips in parallel, with adjacentmicro-strips separated by dielectric material.

The RF coils presented above with respect to FIGS. 2-9 and 11-13 may beutilized in order to receive MR signals during an MR imaging session. Assuch, the RF coils of FIGS. 2-9 and 11-13 may be non-limiting examplesof elements in RF coil unit 14 of FIG. 1 and may be configured to becoupled to a downstream component of the MRI system, such as aprocessing system. The RF coils of FIGS. 2-9 and 11-13 may be present inan array of RF coils having various configurations. FIG. 14 illustratesvarious embodiments for RF coil arrays and accompanying coil-interfacingcables that may include one or more of the RF coils described above withrespect to FIGS. 2-9 and 11-13.

First RF coil array 1310 includes a coil loop and an electronics unitcoupled to each coil loop, and a coil-interfacing cable connected to andextending from each coupling electronics unit. Accordingly, RF coilarray 1310 includes four coil loops, four electronics units, and fourcoil-interfacing cables. For example, one RF coil of RF coil array 1310includes a loop portion 1312, electronics unit 1314, andcoil-interfacing cable 1316. Loop portion 1312 may include a suitablenumber of conductors, such as two conductor wires (similar to the RFcoils illustrated in FIGS. 3 and 4), four conductor wires (similar tothe RF coil loop portion illustrated in FIG. 6), or six conductor wires(similar to the RF coil loop portion illustrated in FIG. 8).

Second RF coil array 1320 includes a separate electronics unit for eachcoil loop, with each electronics unit coupled to a respectivecoil-interfacing cable. Array 1320 includes four coil loops, fourelectronics units, and four coil-interfacing cables that are bundledtogether in a single grouping of four coil-interfacing cables, and maybe referred to as an integrated balun cable harness. For example, thetwo coil-interfacing cables coupled to the two top electronics units arebundled together, and they are bundled with the two coil-interfacingcables from the two bottom electronics units. Similar to the RF coilarray described above, each coil loop of each RF coil in array 1320 mayinclude two or more conductors.

Third RF coil array 1330 includes a separate electronics unit for eachcoil loop, with each electronics unit coupled to a respectivecoil-interfacing cable. Array 1330 includes four coil loops, fourelectronics units, and four coil-interfacing cables that are bundledtogether in a single grouping of four coil-interfacing cables, and maybe referred to as an integrated balun cable harness. Similar to the RFcoil arrays described above, each coil loop of each RF coil in array1330 may include two or more conductors.

The individual coupling electronics units may be housed in a commonelectronics housing in some examples. Each coil loop of the coil arraymay have respective coupling electronics unit (e.g., a decouplingcircuit, impedance inverter circuit, and pre-amplifier) housed in thehousing. In some examples, the common electronics housing may bedetachable from the coil loop or RF coil array. In particular, if theindividual coupling electronics are configured as in the RF coil array1330 of FIG. 11, the electronics may be placed in a separable assemblyand disconnected from the coil array. A connector interface could beplaced at, for example, the junctions between the conductor loopportions (e.g., the drive ends described above) and the couplingelectronics for each individual coupling electronics unit.

The conductor wires and coil loops used in the RF coil or RF coil arraymay be manufactured in any suitable manner to get the desired resonancefrequency for a desired RF coil application. The desired conductor wiregauge, such as 28 or 30 American Wire Gauge (AWG) or any other desiredwire gauge may be paired with one or more parallel conductor wires ofthe same gauge and encapsulated with a dielectric material using anextrusion process or a three-dimensional (3D) printing or additivemanufacturing process. This manufacturing process may be environmentallyfriendly with low waste and low-cost.

Thus, the RF coil described herein includes a twin lead, triplet lead,quadruplet lead, sextuplet lead, or other suitable lead conductor wireloop encapsulated in a pTFE, dielectric with optional cut(s) in one ormore of the parallel conductors and a miniaturized coupling electronicsPCB coupled to each coil loop (e.g., very small coupling electronics PCBapproximately the size of 2 cm² or smaller). The PCBs may be protectedwith a conformal coating or an encapsulation resin. In doing so,traditional components are eliminated and capacitance is “built in” theintegrated capacitor (INCA) coil loops. Interactions between coilelements are reduced or eliminated. The coil loops are adaptable to abroad range of MR operating frequencies by changing the gauge ofconductor wire used, spacing between conductor wires, number ofconductors, loop diameters, loop shapes, dielectric material, loop corematerial, and the number and placement of cuts in the conductor wires.The coil loops are transparent in PET/MR applications, aiding dosemanagement and signal-to-noise ratios (SNR). The miniaturized couplingelectronics PCB includes decoupling circuitry, impedance invertercircuitry with impedance matching circuitry and an input balun, and apre-amplifier. The pre-amplifier sets new standards in coil arrayapplications for lowest noise, robustness, and transparency. Thepre-amplifier provides active noise cancelling to reduce current noise,boost linearity, and improve tolerance to varying coil loadingconditions. Additionally, as explained in more detail below, a cableharness with baluns for coupling each of the miniaturized couplingelectronics PCBs to the RF coil array connector that interfaces with theMRI system may be provided.

The RF coil described herein is exceptionally lightweight, and may weighless than 10 grams per coil element versus 45 grams per coil elementwith General Electric Company's Geometry Embracing Method (GEMS) suiteof flexible RF coil arrays. For example, a 16-channel RF coil arrayaccording to the disclosure may weigh less than 0.5 kg. The RF coildescribed herein is exceptionally flexible and durable as the coil isextremely simple with very few rigid components and allowing floatingoverlaps. The RF coil described herein is exceptionally low-cost, e.g.,greater than ten times reduction from current technology. For example, a16-channel RF coil array could be comprised of components and materialsof less than 550. The RF coil described herein does not preclude currentpackaging or emerging technologies and could be implemented in RF coilarrays that do not need to be packaged or attached to a former, orimplemented in RF coil arrays that are attached to flexible formers asflexible RF coil arrays or attached to rigid formers as rigid RF coilarrays.

The combination of an INCA coil loop and associated coupling electronicsis a single coil element, which is functionally independent andelectrically immune to its surrounding environment or neighboring coilelements. As a result, the RF coil described herein performs equallywell in low and high-density coil array applications. The exceptionalisolation between coil elements allows the overlap between coil elementsto be maximized without degrading performance across coil elements. Thisallows for a higher density of coil elements than is possible withtraditional RF coil array designs.

In contrast to the RF coil arrays described herein, a traditional RFcoil array may include four RF coil loops that include copper traced onPCB, which is rigid and maintains the RF coils at fixed positionsrelative to each other. Traditional RF coils include lumped elements(e.g., capacitors) and a relatively large set of coupling electronics,as compared to the electronics of the RF coil arrays described herein.For example, a traditional RF coil array includes a PCB on which coppertraces are formed and lumped elements are present. A set of couplingelectronics may include bulky and rigid elements, such as a balun.Further, due to the configuration of the traditional RF coil array(e.g., due to heat generation by the coil array), a rigid and/or bulkyhousing material is required. Further, the traditional RF coil array maycomprise only a portion of a traditional overall RF coil array elementactually used during MR imaging. For example, a traditional overall RFcoil array element may include four separate traditional RF coil arrays,further increasing the size, weight, and cost of a traditional overallRF coil array element.

The RF coils described herein are not supported by or surrounded by asubstrate. While the wires in the RF coil loops are encapsulated indielectric material, at least in some examples, no other substrates arepresent around the entirety of the RF coils. The RF coils may be housedin fabric or other flexible enclosures, but the RF coils may remainflexible in multiple dimensions and may not be fixedly connected to oneanother. In some examples, the RF coils may be slidably movable relativeto each other, such that varying amounts of overlap among coil elementsis provided. In contrast, the coil elements of a traditional RF arrayare fixed in position relative to each other and are surrounded bysubstrate (e.g., PCB). Thus, even when the substrate is flexible, themovement of the coil elements of a traditional RF coil array is limited.

The image quality generated by signals received by an RF coil may bedefined by a quality factor, referred as Q. In an RF coil, Q is definedas Q=ωL/R where L is the coil inductance, R is the circuit resistance,and ω is the angular frequency. An increasing Q results in an increasedSNR and a sharper frequency response. The Q value is inversely relatedto the fraction of energy in an oscillation system lost in oscillationcycle. Accordingly, to increase SNR, it may be desirable to generate anRF coil array with a relatively high Q value. Q may be determined duringan unloaded state where no patient is present during imaging, which isreferred to as the unloaded Q of an RF coil. The RF coils describedherein may have varying unloaded Q depending on the number of conductorspresent. For example, twin lead RF coils (such as those described abovewith respect to FIGS. 3 and 4) may have relatively low unloaded Q, whichmay be lower than the unloaded Q of a traditional copper trace RF coil.In contrast, the RF coils that include an increased number ofconductors, such as the RF coil described above with respect to FIG. 8that includes six conductors, may have an unloaded Q similar to theunloaded Q of a traditional copper trace RF coil.

As mentioned previously, the RF coil array of the present disclosure maybe coupled to a RF coil array interfacing cable that includescontiguous, distributed baluns or common-mode traps in order to minimizehigh currents or standing waves, independent of positioning. High stressareas of the RF coil array interfacing cable may be served by severalbaluns. Additionally, the thermal load may be shared through a commonconductor. The inductance of the central path and return path of the RFcoil array interfacing cables are not substantially enhanced by mutualinductance, and therefore are stable with geometry changes. Capacitanceis distributed and not substantially varied by geometry changes.Resonator dimensions are ideally very small, but in practice may belimited by blocking requirements, electric and magnetic fieldintensities, local distortions, thermal and voltage stresses, etc.

FIG. 15 illustrates a block schematic diagram of a continuous commonmode trap assembly 1400 formed in accordance with various embodiments.The common mode trap assembly 1400 may be configured, for example, foruse in an MRI system, such as the MRI apparatus 10 described hereinabove. For example, in the illustrated embodiment, the common mode trapassembly 1400 is configured as a transmission cable 1401 configured fortransmission of signals between a processing system 1450 and a RF coilarray 1460 of an MRI system. Transmission cable 1401 is a non-limitingexample of a RF coil array interfacing cable 212, processing system 1450is a non-limiting example of controller unit 210, and RF coil array 1460is a non-limiting example of a plurality of RF coils 202 and couplingelectronics portion 203 of FIG. 2.

In the illustrated embodiment, the transmission cable 1401 (or RF coilarray interfacing cable) includes a central conductor 1410 and pluralcommon mode traps 1412, 1414, 1416. It may be noted that, while thecommon mode traps 1412, 1414, and 1416 are depicted as distinct from thecentral conductor 1410, in some embodiments, the common mode traps 1412,1414, 1416 may be integrally formed with or as a part of the centralconductor 1410.

The central conductor 1410 in the illustrated embodiment has a length1404, and is configured to transmit a signal between the MRI RF coilarray 1460 and at least one processor of an MRI system (e.g., processingsystem 1450). The central conductor 1410 may include one or more of aribbon conductor, a wire, or a coaxial cable bundle, for example. Thelength 1404 of the depicted central conductor 1410 extends from a firstend of the central conductor 1410 (which is coupled to the processingsystem 1450) to a second end of the central conductor 1410 (which iscoupled to the RF coil array 1460). In some embodiments, the centralconductor may pass through a central opening of the common mode traps1412, 1414, 1416.

The depicted common mode traps 1412, 1414, 1416 (which may be understoodas cooperating to form a common mode trap unit 1418), as seen in FIG.15, extend along at least a portion of the length 1404 of the centralconductor 1410. In the illustrated embodiment, common mode traps 1412,1414, 1416 do not extend along the entire length 1404. However, in otherembodiments, the common mode traps 1412, 1414, 1416 may extend along theentire length 1404, or substantially along the entire length 1404 (e.g.,along the entire length 1404 except for portions at the end configuredto couple, for example, to a processor or RF coil array). The commonmode traps 1412, 1414, 1416 are disposed contiguously. As seen in FIG.15, each of the common mode traps 1412, 1414, 1416 is disposedcontiguously to at least one other of the common mode traps 1412, 1414,1416. As used herein, contiguous may be understood as includingcomponents or aspects that are immediately next to or in contact witheach other. For example, contiguous components may be abutting oneanother. It may be noted that in practice, small or insubstantial gapsmay be between contiguous components in some embodiments. In someembodiments, an insubstantial gap (or conductor length) may beunderstood as being less than 1/40^(th) of a wavelength of a transmitfrequency in free space. In some embodiments, an insubstantial gap (orconductor length) may be understood as being two centimeters or less.Contiguous common mode traps, for example, have no (or insubstantial)intervening gaps or conductors therebetween that may be susceptible toinduction of current from a magnetic field without mitigation providedby a common mode trap.

For example, as depicted in FIG. 15, the common mode trap 1412 iscontiguous to the common mode trap 1414, the common mode trap 1414 iscontiguous to the common mode trap 1412 and the common mode trap 1416(and is interposed between the common mode trap 1412 and the common modetrap 1416), and the common mode trap 1416 is contiguous to the commonmode trap 1414. Each of the common mode traps 1412, 1414, 1416 areconfigured to provide an impedance to the receive transmitter drivencurrents of an MRI system. The common mode traps 1412, 1414, 1416 invarious embodiments provide high common mode impedances. Each commonmode trap 1412, 1414, 1416, for example, may include a resonant circuitand/or one or more resonant components to provide a desired impedance ator near a desired frequency or within a target frequency range. It maybe noted that the common mode traps 1412, 1414, 1416 and/or common modetrap unit 1418 may also be referred to as chokes or baluns by thoseskilled in the art.

In contrast to systems having separated discrete common mode traps withspaces therebetween, various embodiments (e.g., the common mode trapassembly 1400) have a portion over which common mode traps extendcontinuously and/or contiguously, so that there are no locations alongthe portion for which a common mode trap is not provided. Accordingly,difficulties in selecting or achieving particular placement locations ofcommon mode traps may be reduced or eliminated, as all locations ofinterest may be included within the continuous and/or contiguous commonmode trap. In various embodiments, a continuous trap portion (e.g.,common mode trap unit 1418) may extend along a length or portion thereofof a transmission cable. The continuous mode trap portion may be formedof contiguously-joined individual common mode traps or trap sections(e.g., common mode traps 1412, 1414, 1416). Further, contiguous commonmode traps may be employed in various embodiments to at least one oflower the interaction with coil elements, distribute heat over a largerarea (e.g., to prevent hot spots), or help ensure that blocking islocated at desired or required positions. Further, contiguous commonmode traps may be employed in various embodiments to help distributevoltage over a larger area. Additionally, continuous and/or contiguouscommon mode traps in various embodiments provide flexibility. Forexample, in some embodiments, common mode traps may be formed using acontinuous length of conductor(s) (e.g., outer conductors wrapped abouta central conductor) or otherwise organized as integrally formedcontiguous sections. In various embodiments, the use of contiguousand/or continuous common mode traps (e.g., formed in a cylinder) providefor a range of flexibility over which flexing of the assembly does notsubstantially change the resonant frequency of the structure, or overwhich the assembly remains on frequency as it is flexed.

It may be noted that the individual common mode traps or sections (e.g.,common mode traps 1412, 1414, 1416) in various embodiments may beconstructed or formed generally similarly to each other (e.g., each trapmay be a section of a length of tapered wound coils), but eachindividual trap or section may be configured slightly differently thanother traps or sections. For example, in some embodiments, each commonmode trap 1412, 1414, 1416 is tuned independently. Accordingly, eachcommon mode trap 1412, 1414, 1416 may have a resonant frequency thatdiffers from other common mode traps of the same common mode trapassembly 1400.

Alternatively or additionally, each common mode trap may be tuned tohave a resonant frequency near an operating frequency of the MRI system.As used herein, a common mode trap may be understood as having aresonant frequency near an operating frequency when the resonantfrequency defines or corresponds to a band that includes the operatingfrequency, or when the resonant frequency is close enough to theoperating frequency to provide on-frequency blocking, or to provide ablocking impedance at the operating frequency.

Further additionally or alternatively, each common mode trap may betuned to have a resonant frequency below an operating frequency of theMRI system (or each common mode trap may be tuned to have resonantfrequency above an operating frequency of the MRI system). With eachtrap having a frequency below (or alternatively, with each trap having afrequency above) the operating frequency, the risk of any of the trapscanceling each other out (e.g., due to one trap having a frequency abovethe operating frequency and a different trap having a frequency belowthe operating frequency) may be eliminated or reduced. As anotherexample, each common mode trap may be tuned to a particular band toprovide a broadband common mode trap assembly.

In various embodiments, the common mode traps may have a two (2D) orthree-dimensional (3D) butterfly configuration to counteract magneticfield coupling and/or local distortions.

FIG. 16 is a perspective view of a RF coil array interfacing cable 1500including a plurality of continuous and/or contiguous common mode trapsaccording to an embodiment of the disclosure. The RF coil arrayinterfacing cable 1500 includes an outer sleeve or shield 1503, adielectric spacer 1504, an inner sleeve 1505, a first common mode trapconductor 1507, and a second common mode trap conductor 1509.

The first common mode trap conductor 1507 is wrapped in a spiral aboutthe dielectric spacer 1504, or wrapped in a spiral at a taperingdistance from a central conductor (not shown) disposed within the bore1518 of the RF coil array interfacing cable 1500, in a first direction1508. Further, the second common mode trap conductor 1509 is wrapped ina spiral about the dielectric spacer 1504, or wrapped in a spiral at atapering distance from the central conductor disposed within the bore1518, in a second direction 1510 that is opposite to the first direction1508. In the illustrated embodiment, the first direction 1508 isclockwise and the second direction 1510 is counter-clockwise.

The conductors 1507 and 1509 of the RF coil array interfacing cable 1500may comprise electrically-conductive material (e.g., metal) and may beshaped as ribbons, wires, and/or cables, for example. In someembodiments, the counterwound or outer conductors 1507 and 1509 mayserve as a return path for a current passing through the centralconductor. Further, in various embodiments, the counterwound conductors1507 and 1509 may cross each other orthogonally (e.g., a center line orpath defined by the first common mode trap conductor 1507 isperpendicular to a center line or path defined by the second common modetrap conductor 1509 as the common mode trap conductors cross paths) toeliminate, minimize, or reduce coupling between the common mode trapconductors.

It may be further noted that in various embodiments the first commonmode trap conductor 1507 and the second common mode trap conductor 1509are loosely wrapped about the dielectric spacer 1504 to provideflexibility and/or to reduce any binding, coupling, or variation ininductance when the RF coil array interfacing cable 1500 is bent orflexed. It may be noted that the looseness or tightness of thecounterwound outer conductors may vary by application (e.g., based onthe relative sizes of the conductors and dielectric spacer, the amountof bending or flexing that is desired for the common mode trap, or thelike). Generally, the outer or counterwound conductors should be tightenough so that they remain in the same general orientation about thedielectric spacer 1504, but loose enough to allow a sufficient amount ofslack or movement during bending or flexing of the RF coil arrayinterfacing cable 1500 to avoid, minimize, or reduce coupling or bindingof the counterwound outer conductors.

In the illustrated embodiment, the outer shielding 1503 is discontinuousin the middle of the RF coil array interfacing cable 1500 to expose aportion of the dielectric spacer 1504 which in some embodiments isprovided along the entire length of the RF coil array interfacing cable1500. The dielectric spacer 1504, may be comprised, as a non-limitingexample, of Teflon or another dielectric material. The dielectric spacer1504 functions as a capacitor and thus may be tuned or configured toprovide a desired resonance. It should be appreciated that otherconfigurations for providing capacitance to the RF coil arrayinterfacing cable 1500 are possible, and that the illustratedconfigurations are exemplary and non-limiting. For example, discretecapacitors may alternatively be provided to the RF coil arrayinterfacing cable 1500.

Further, the RF coil array interfacing cable 1500 includes a first post1513 and a second post (not shown) to which the first common mode trapconductor 1507 and the second common mode trap conductor 1509 are fixed.To that end, the first post 1513 and the second post are positioned atthe opposite ends of the common mode trap, and are fixed to the outershielding 1503. The first post 1513 and the second post ensure that thefirst and second common mode trap conductors 1507 and 1509 arepositioned close to the outer shielding 1503 at the ends of the RF coilarray interfacing cable 1500, thereby providing a tapered butterflyconfiguration of the counterwound conductors as described furtherherein.

The tapered butterfly configuration includes a first loop formed by thefirst common mode trap conductor 1507 and a second loop formed by thesecond common mode trap conductor 1509, arranged so that an inducedcurrent (a current induced due to a magnetic field) in the first loop1507 and an induced current in the second loop 1509 cancel each otherout. For example, if the field is uniform and the first loop 1507 andthe second loop 1509 have equal areas, the resulting net current will bezero. The tapered cylindrical arrangement of the loops 1507 and 1509provide improved flexibility and consistency of resonant frequencyduring flexing relative to two-dimensional arrangements conventionallyused in common mode traps.

Generally, a tapered butterfly configuration as used herein may be usedto refer to a conductor configuration that is flux cancelling, forexample including at least two similarly sized opposed loops that aresymmetrically disposed about at least one axis and are arranged suchthat currents induced in each loop (or group of loops) by a magneticfield tends to cancel out currents induced in at least one other loop(or group of loops). For example, with reference to FIG. 15, in someembodiments, counterwound conductors (e.g., conductors wound about acentral member and/or axis in opposing spiral directions) may be spaceda distance radially from the central conductor 1410 to form the commonmode traps 1412, 1414, 1416. As depicted in FIG. 16, the radial distancemay be tapered towards the end of the common mode traps to reduce oraltogether eliminate fringe effects. In this way, the common mode traps1412, 1414, 1416 may be continuously or contiguously positioned withoutsubstantial gaps therebetween.

The tapered spiral configuration of the common mode trap conductorsdescribed herein above is particularly advantageous when multiple commonmode trap conductors are contiguously disposed in a common mode trapassembly. As an illustrative example, FIG. 17 is a perspective view of aRF coil array interfacing cable 1550 including a plurality of continuousand/or contiguous common mode traps coupling an RF coil array 1570 to aprocessing system 1560. RF coil array interfacing cable 1550 includes afirst common mode trap 1580 and a second common mode trap 1590positioned adjacent to each other on a central conductor 1552.

The first common mode trap 1580 includes a first common mode trapconductor 1582 and a second common mode trap conductor 1584 counterwoundin a tapered spiral configuration. To that end, the first and secondconductors 1582 and 1584 are fixed to posts 1586 and 1588. It should benoted that the posts 1586 and 1588 are aligned on a same side of thecommon mode trap 1580.

Similarly, the second common mode trap 1590 includes a third common modetrap conductor 1592 and a fourth common mode trap conductor 1594counterwound in a tapered spiral configuration and fixed to posts 1596and 1598. It should be noted that the posts 1596 and 1598 are aligned ona same side of the common mode trap 1590.

As depicted, the common mode traps 1580 and 1590 are separated by adistance, thereby exposing the central conductor 1552 in the gap 1554between the common mode traps. Due to the tapering spiral configurationof the common mode trap conductors of the common mode traps, the gap1554 can be minimized or altogether eliminated in order to increase thedensity of common mode traps in a common mode trap assembly without lossof impedance functions of the common mode traps. That is, the distancecan be made arbitrarily small such that the common mode traps are inface-sharing contact, given the tapered spiral configuration.

It should be appreciated that while the RF coil array interfacing cable1550 includes two common mode traps 1580 and 1590, in practice a RF coilarray interfacing cable may include more than two common mode traps.

Further, the common mode traps 1580 and 1590 of the RF coil arrayinterfacing cable 1550 are aligned such that the posts 1586, 1588, 896,and 1598 are aligned on a same side of the RF coil array interfacingcable. However, in examples where cross-talk between the common modetraps may be possible, for example if the tapering of the counterwoundconductors is more severe or steep, the common mode traps may be rotatedwith respect to one another to further reduce fringe effects and/orcross-talk between the traps.

Additionally, other common mode trap or balun configurations arepossible. For example, the exterior shielding of each common mode trapmay be trimmed such that the common mode traps can be overlapped orinterleaved, thus increasing the density of the common mode traps.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements or a particular positional order on their objects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

The invention claimed is:
 1. A radio frequency (RF) coil assembly for a magnetic resonance imaging (MRI) system, comprising: a loop portion comprising at least three distributed capacitance conductor wires encapsulated and separated by a dielectric material; a coupling electronics portion including a pre-amplifier; and a coil-interfacing cable extending between the coupling electronics portion and an interfacing connector of the RF coil assembly, wherein the loop portion comprises four parallel conductor wires encapsulated and separated by the dielectric material, the four parallel conductor wires arranged in a square configuration, and wherein current flows through a first pair of the four parallel conductor wires in an ascending manner and current flows through a second pair of the four parallel conductor wires in a descending manner, the first pair comprising every other parallel conductor wire of the square configuration and the second pair comprising every intervening parallel conductor wire of the square configuration.
 2. The RF coil assembly of claim 1, wherein the loop portion further includes a core surrounded by the at least three distributed capacitance conductor wires.
 3. The RF coil assembly of claim 2, wherein the core is made of a second dielectric material.
 4. The RF coil assembly of claim 1, wherein the coupling electronics portion further includes a decoupling circuit configured to decouple the RF coil assembly during a transmit operation and an impedance inverter circuit.
 5. A radio frequency (RF) coil assembly of claim 4, for a magnetic resonance imaging (MRI) system, comprising: a loop portion comprising at least three distributed capacitance conductor wires encapsulated and separated by a dielectric material; a coupling electronics portion including a pre-amplifier; and a coil-interfacing cable extending between the coupling electronics portion and an interfacing connector of the RF coil assembly, wherein the loop portion comprises six parallel conductor wires encapsulated and separated by the dielectric material, the six parallel conductor wires arranged in a hexagonal configuration, and wherein current flows through a first subset of the six parallel conductor wires in an ascending manner and current flows through a second subset of the six parallel conductor wires in a descending manner, the first subset comprising every other parallel conductor wire of the hexagonal configuration and the second subset comprising every intervening parallel conductor wire of the hexagonal configuration.
 6. The RF coil assembly of claim 5, wherein the loop portion further includes a core surrounded by the at least three distributed capacitance conductor wires.
 7. The RF coil assembly of claim 6, wherein the core is made of a second dielectric material.
 8. The RF coil assembly of claim 5, wherein the coupling electronics portion further includes a decoupling circuit configured to decouple the RF coil assembly during a transmit operation and an impedance inverter circuit.
 9. A radio frequency (RF) coil assembly for a magnetic resonance imaging (MRI) system, comprising: a loop portion comprising at least three distributed capacitance conductor wires encapsulated and separated by a dielectric material; a coupling electronics portion including a pre-amplifier; and a coil-interfacing cable extending between the coupling electronics portion and an interfacing connector of the RF coil assembly, wherein the loop portion comprises at least three coaxial conductive wires encapsulated and separated by the dielectric material, and wherein the at least three coaxial conductor wires include an inner shield surrounding a core, a first outer shield surrounding the inner shield, and a second outer shield surrounding the first outer shield.
 10. The RF coil assembly of claim 9, wherein the loop portion includes at least three parallel planar strips encapsulated and separated by the dielectric material.
 11. The RF coil assembly of claim 9, wherein the loop portion further includes a core surrounded by the at least three distributed capacitance conductor wires.
 12. The RF coil assembly of claim 11, wherein the core is made of a second dielectric material.
 13. A radio frequency (RF) coil array assembly for a magnetic resonance imaging (MRI) system, comprising: a plurality of RF coils, each RF coil comprising: a loop portion comprising at least three distributed capacitance conductor wires encapsulated and separated by a dielectric material; and a coupling electronics portion including a pre-amplifier; and a coil-interfacing cable extending between the coupling electronics portion and an interfacing connector of the RF coil array assembly, wherein the loop portion of each RF coil comprises at least three coaxial conductive wires encapsulated and separated by the dielectric material, and wherein the at least three coaxial conductor wires include an inner shield surrounding a core, a first outer shield surrounding the inner shield, and a second outer shield surrounding the first outer shield.
 14. The RF coil array assembly of claim 13, wherein the plurality of RF coils are movable relative to each other.
 15. The RF coil array assembly of claim 13, wherein the loop portion of each RF coil comprises four parallel conductor wires encapsulated and separated by the dielectric material, the four parallel conductor wires arranged in a square configuration.
 16. The RF coil array assembly of claim 13, wherein the loop portion of each RF coil comprises six parallel conductor wires encapsulated and separated by the dielectric material, the six parallel conductor wires arranged in a hexagonal configuration.
 17. The RF coil array assembly of claim 13, wherein the loop portion of each RF coil comprises at least three parallel planar strips encapsulated and separated by the dielectric material. 